Sauron forged the One Ring in the fires of Mount Doom.
During the Second Age, Sauron created the Ring in the volcanic fire of Orodruin, also called Mount Doom, in Mordor. He poured much of his own power into it so that it could dominate the other Rings of Power and help him control their bearers.
The Elven-smith Celebrimbor and the craftsmen of Eregion made the other great Rings, but Sauron secretly forged the One Ring himself. Its power was therefore closely bound to his survival and authority. Destroying it in the same volcanic fire was necessary to defeat him permanently.
A common mix-up identifies Celebrimbor as the maker of the One Ring. Celebrimbor forged the Three Rings of the Elves, while Sauron made the ruling Ring. The Ring’s inscription is written in Tolkien’s Black Speech of Mordor.